This trial aims to investigate the effect of cells from bone marrow on osteoarthritis of the knee. The trial is being undertaken at two centres by Centro Medico Tecnon in Barcelona and Hospital Clinico Universitario in Valladolid, in conjunction with a biotechnology firm Red Terapia de Cellular. The trial is designed to recruit a total of 30 subjects from April 2012 to December 2013 with documented Grade II, III and IV osteoarthritis. Participants will be enrolled in either the control arm where they will receive injections into the joint (intra-articular injections) of hyaluronic acid; or they will be enrolled in the treatment arm where they will receive injections into the joint of a preparation of bone marrow mesenchymal stem cells. The cells that have been harvested from healthy donors and expanded in the laboratory. The participants will be monitored for complications and any improvement of their osteoarthritis using standard assessments including MRI imaging of cartilage thickness.